1. Field of the Invention
The present invention relates to a fastening structure for fuel cell stack made up from a plurality of a fuel cells each comprising an anode electrode and a cathode electrode which are disposed in facing relation to each other with an electrolyte interposed therebetween. The fuel cells are stacked with separators interposed therebetween and end plates disposed on respective opposite ends of the plurality of fuel cells.
2. Description of the Related Art
A known type of fuel cell stack comprises a plurality of fuel cells alternating with separators, wherein each of the fuel cells comprises an anode electrode and a cathode electrode which are disposed in facing relation to each other with a solid polymer electrolyte membrane interposed therebetween. The known fuel cell stack operates as follows: A hydrogen containing gas, i.e., a fuel gas, supplied to the anode electrode is converted into hydrogen ions on a catalytic electrode, and the hydrogen ions move through the solid polymer electrolyte membrane, which is appropriately humidified, toward the cathode electrode. Electrons which are generated in connection with the movement of the hydrogen ions are supplied to an external circuit for use as DC electric energy. Since the cathode electrode is supplied with an oxidizing gas such as an oxygen gas or air, the hydrogen ions, the electrons, and the oxygen react to form water on the cathode electrode.
If the contact resistance in the fuel cells increases, then the internal resistance causes an increased loss, resulting in a drop in the voltage across the fuel cell stack. Therefore, it is necessary to exert predetermined fastening forces on the fuel cells, for thereby applying a uniform pressure to the electrode surfaces, in order to reduce the contact resistance.
Japanese laid-open patent publication No. 58-164168, for example, discloses a fuel cell stack which comprises stacked cell blocks each comprising a stack of unit cells alternating with separator plates, rigid intermediate plates interposed between the cell blocks, and a pair of end plates placed on respective opposite ends of the cell block stack. The stacked cell blocks are fastened together by bolts extending through the separator plates and the end plates. Sealed receptacles that are filled with a liquid are inserted between the intermediate and end plates and the cell blocks. The sealed receptacles are of a size large enough to cover the entire end surfaces of the corresponding cell blocks and have flexible end walls held in contact with those cell blocks. The sealed receptacles are held in fluid communication with each other by conduits.
However, the disclosed fuel cell stack suffers a disadvantage in that since the liquid-filled sealed receptacles are inserted between the intermediate and end plates and the cell blocks, passages for a hydrogen containing gas, an oxidizing gas, and a coolant to be supplied to the cell blocks cannot be defined in the intermediate and end plates and the cell blocks. Therefore, the fuel cell stack needs to have additional passages, extending around the intermediate and end plates and the cell blocks, for passing a hydrogen containing gas, an oxidizing gas, and a coolant. As a result, the fuel cell stack is relatively complex in its overall structure, and cannot be reduced in weight and made compact.
Furthermore, the cell blocks are associated with the respective liquid-filled sealed receptacles which communicate with each other. Consequently, it is not possible to control the fastening forces individually for the respective cell blocks.